The availability of computer tomography and specialised computer software allows for detailed pre-operative planning, during which the precise location and dimensions of the required cavity in a patient's jaw bone can be virtually modelled. Based on this model, a template specific to each patient can be designed and manufactured, which can be placed over the patient's teeth, gums or bone. This template contains one or more guide channels precisely positioned along the axis of the bone cavities to be drilled. During their use, all surgical tools required for creating the cavity and guiding the implant are inserted through this guide channel to ensure the correct alignment of the tools and implant with respect to the bone. This necessitates a close fit between the guide channel and the body of each tool. So that the guide channel can accurately guide tools having a smaller diameter than the channel, a series of guide channel collars can be provided. These are placed over and/or in the guide channel and effectively reduce its diameter to correspond to the tool in use. A surgical procedure with some kind of guide element, for example the template and collars described above, is herein referred to as guided surgery.
The precise guidance of surgical tools is very important during all steps of guided surgery. The use of a template constitutes a great facilitation with respect to correct positioning and alignment of the implant and for precise handling of the tools. In addition, it can also be used to assist in the control of tool and implant insertion depth.
Depth control can be achieved via physical or visual means, or a combination of both. The type used will depend on user preference but is also influenced by tool function.
During drilling, for instance, depth information is crucial in order to precisely control the depth of the drill head. This avoids damaging, e.g. of underlying nerves and other anatomical structures. For this reason, a physical stop is often provided to prevent over-drilling. For example, the drill body can undergo a step increase in diameter at a certain distance from the drill tip. This portion of increased diameter can not pass through the guide channel and therefore will limit the axial feed of the drill. In addition specific tools have been developed to limit the axial feed of drilling devices during implant bed preparation. These tools can be applied with or without the aid of a template.
WO 2006/062459 describes a dental drill device to which sleeves of different lengths can be attached. Towards the drill head, the sleeves have bearing surfaces, which, in cooperation with the bone, template or other guide element, can be used to limit the drill length or the drill depth.
In addition, or alternatively, visual indicators can be provided on the drill head or body to inform the surgeon how deep the drill has penetrated.
By providing tools with only visual depth indicators the flexibility available to the dentist or surgeon is increased, as the tool or implant may be inserted to a deeper depth than initially planned.
Such visual indications are particularly useful for steps which do not require meticulous depth control, during flattening of the alveolar ridge with a milling cutter, for example, or removing mucosa with a punch. In such cases, sophisticated technical features to precisely determine the position of the functional element of the tool or the use of physical stops are not necessary.
Physical stops are also not always desirable from an operative viewpoint. For example, a tap is used in dentistry to cut threads into the bone cavity walls. Said threads possess a pitch equal to that of the implant to be inserted. If the tap is brought to a sudden vertical stop whilst continuing to rotate, the threads created within the implant hole will be destroyed. The same problem exists in respect of the implants themselves, which are inserted into the cavity using a transfer piece or implant post.
In this context, tools for use in guided surgery have been developed that cooperate with the template or other guide element to provide visual information on insertion depth. These tools comprise a guide portion, which, while in use, is in contact with the guide element. Said guide portion features one or several marks at defined distances from a reference point of the functional element, for example the tool tip or the start of the cutting edge. These marks, together with the template or other guide element, help the practitioner to ascertain the axial feed of the tool.
Laser-marking is a known method of providing tools with visual marks. These marks are created at one or more set distances from the functional element of the tool. During the process of laser-marking, the irradiated material melts and the corresponding surface of the tool slightly deforms. This deformation results in a slight increase in diameter of a cylindrical tool. The amount of deformation is dependent on the energy used during the laser-marking process, however typically this is greater than 0.01 mm. Due to the close fit of the tool with the guide channel, said diameter increase can prevent smooth guidance of the tool. Additionally, due to the rotation and axial movement of the tool during usage, the laser marks are quickly abraded. As a result thereof, the practitioner's ability to track the axial feed of the tool is diminished. Similar problems are experienced with other forms of marking, e.g. painting etc.